Wind turbine blades are often manufactured according to one of two constructional designs, namely a design where a thin aerodynamic shell is glued onto a spar beam, or a design where spar caps, also called main laminates, are integrated into the aerodynamic shell.
In the first design, the spar beam constitutes the load bearing structure of the blade. The spar beam as well as the aerodynamic shell or shell parts are manufactured separately. The aerodynamic shell is often manufactured as two shell parts, typically as a pressure side shell part and a suction side shell part. The two shell parts are glued or otherwise connected to the spar beam and are further glued to each other along a leading edge and trailing edge of the shell parts. This design has the advantage that the critical load carrying structure may be manufactured separately and therefore easier to control. Further, this design allows for various different manufacturing methods for producing the beam, such as moulding and filament winding.
In the second design, the spar caps or main laminates are integrated into the shell and are moulded together with the aerodynamic shell. The main laminates typically comprise a high number of fibre layers compared to the remainder of the blade and may form a local thickening of the wind turbine shell, at least with respect to the number of fibre layers. Thus, the main laminate may form a fibre insertion in the blade. In this design, the main laminates constitute the load carrying structure. The blade shells are typically designed with a first main laminate integrated in the pressure side shell part and a second main laminate integrated in the suction side shell part. The first main laminate and the second main laminate are typically connected via one or more shear webs, which for instance may be C-shaped or I-shaped. For very long blades, the blade shells further along at least a part of the longitudinal extent comprise an additional first main laminate in the pressure side shell, and an additional second main laminate in the suction side shell. These additional main laminates may also be connected via one or more shear webs. This design has the advantage that it is easier to control the aerodynamic shape of the blade via the moulding of the blade shell part.
Vacuum infusion or VARTM (vacuum assisted resin transfer moulding) is one method, which is typically employed for manufacturing composite structures, such as wind turbine blades comprising a fibre reinforced matrix material.
During the process of filling the mould, a vacuum, said vacuum in this connection being understood as an under-pressure or negative pressure, is generated via vacuum outlets in the mould cavity, whereby liquid polymer is drawn into the mould cavity via the inlet channels in order to fill said mould cavity. From the inlet channels the polymer disperses in all directions in the mould cavity due to the negative pressure as a flow front moves towards the vacuum channels. Thus, it is important to position the inlet channels and vacuum channels optimally in order to obtain a complete filling of the mould cavity. Ensuring a complete distribution of the polymer in the entire mould cavity is, however, often difficult, and accordingly this often results in so-called dry spots, i.e. areas with fibre material not being sufficiently impregnated with resin. Thus dry spots are areas where the fibre material is not impregnated, and where there can be air pockets, which are difficult or impossible to remove by controlling the vacuum pressure and a possible overpressure at the inlet side. In vacuum infusion techniques employing a rigid mould part and a resilient mould part in the form of a vacuum bag, the dry spots can be repaired after the process of filling the mould by puncturing the bag in the respective location and by drawing out air for example by means of a syringe needle. Liquid polymer can optionally be injected in the respective location, and this can for example be done by means of a syringe needle as well. This is a time-consuming and tiresome process. In the case of large mould parts, staff have to stand on the vacuum bag. This is not desirable, especially not when the polymer has not hardened, as it can result in deformations in the inserted fibre material and thus in a local weakening of the structure, which can cause for instance buckling effects.
In most cases the polymer or resin applied is polyester, vinyl ester or epoxy, but may also be PUR or pDCPD, and the fibre reinforcement is most often based on glass fibres or carbon fibres. Epoxies have advantages with respect to various properties, such as shrinkage during curing (which in some circumstances may lead to less wrinkles in the laminate), electrical properties and mechanical and fatigue strengths. Polyester and vinyl esters have the advantage that they provide better bonding properties to gelcoats.
Thereby, a gelcoat may be applied to the outer surface of the shell during the manufacturing of the shell by applying a gelcoat to the mould before fibre-reinforcement material is arranged in the mould. Thus, various post-moulding operations, such as painting the blade, may be avoided. Further, polyesters and vinyl esters are cheaper than epoxies. Consequently, the manufacturing process may be simplified and costs may be lowered.
Often the composite structures comprise a core material covered with a fibre reinforced material, such as one or more fibre reinforced polymer layers. The core material can be used as a spacer between such layers to form a sandwich structure and is typically made of a rigid, lightweight material in order to reduce the weight of the composite structure. In order to ensure an efficient distribution of the liquid resin during the impregnation process, the core material may be provided with a resin distribution network, for instance by providing channels or grooves in the surface of the core material.
As for instance blades for wind turbines have become bigger and bigger in the course of time and may now be more than 70 meters long, the impregnation time in connection with manufacturing such blades has increased, as more fibre material has to be impregnated with polymer. Furthermore the infusion process has become more complicated, as the impregnation of large shell members, such as blades, requires control of the flow fronts to avoid dry spots, said control may e.g. include a time-related control of inlet channels and vacuum channels. This increases the time required for drawing in or injecting polymer. As a result the polymer has to stay liquid for a longer time, normally also resulting in an increase in the curing time.
Resin transfer moulding (RTM) is a manufacturing method, which is similar to VARTM. In RTM the liquid resin is not drawn into the mould cavity due to a vacuum generated in the mould cavity. Instead the liquid resin is forced into the mould cavity via an overpressure at the inlet side.
Prepreg moulding is a method in which reinforcement fibres are pre-impregnated with a pre-catalysed resin. The resin is typically solid or near-solid at room temperature. The prepregs are arranged by hand or machine onto a mould surface, vacuum bagged and then heated to a temperature, where the resin is allowed to reflow and eventually cured. This method has the main advantage that the resin content in the fibre material is accurately set beforehand. The prepregs are easy and clean to work with and make automation and labour saving feasible. The disadvantage with prepregs is that the material cost is higher than for non-impregnated fibres. Further, the core material need to be made of a material, which is able to withstand the process temperatures needed for bringing the resin to reflow. Prepreg moulding may be used both in connection with a RTM and a VARTM process.
Further, it is possible to manufacture hollow mouldings in one piece by use of outer mould parts and a mould core. Such a method is for instance described in EP 1 310 351 and may readily be combined with RTM, VARTM and prepreg moulding.
Further, it is known to manufacture blades with two or more different types of fibre material. WO 2003/078832 discloses a wind turbine blade of fibre-reinforced polymer including a first type of fibres, such as glass fibres, of a first stiffness and a second type of fibres, such as carbon fibres, of a different stiffness. In a transition region between the two types of fibres the quantitative ratio of the two types of fibres varies continuously in the longitudinal direction of the blade. In a described preferred embodiment, the laminate comprises a plurality of layers, and the boundaries between layers having the first types of fibres and layers having the second types of fibres are mutually shifted in the longitudinal direction of the blade so that a step tapered transition is achieved. However, it has been found that such a transition is not mechanically strong. In order to compensate for stress concentrations when using reinforcement fibres with differing E modulus in composites, it is possible to provide a local thickening in the transition area between the two different fibres and thereby limit the risk of failure due to stress concentrations. One drawback of such a solution is, however, increased weight due to the increased use of fibres, e.g. glass fibres, in the transition area between glass fibres and carbon fibres.
US 2012/0009070 discloses a method of preparing a wind turbine blade shell member by use of pre-cured fibre-reinforced sheet material. In one embodiment, a step infusion process is described, where individual layers are infused in sequence.
WO 2012/149939 discloses a method of preparing a hybrid composite laminate of layers of fibre reinforced material of different resin viscosities, where first layers are pre-impregnated with a first resin having a first viscosity, and second layers are impregnated with a resin of a second viscosity.
WO 2013/010979 discloses a wind turbine blade having different types of fibres with a scarf-like transition between the different fibre types.
US 2012/0082558 discloses a modular wind turbine blade, where parts are bonded to each other along bond lines. In one embodiment, bond lines are formed as double scarf joints.
Additionally, it is known from WO 2013/113817 to manufacture a blade shell part in a mould system and transferring the cured blade shell parts to a post-moulding station comprising cradles for carrying the cured shell parts for additional treatment, such as gluing the shell parts together to form the aerodynamic shell of a finished wind turbine blade. The method ensures that the mould cycle time is kept as short as possible allowing for maximum effective mould usage. The method is particular suitable for blades having a length of 40 to 50 meters, since the layup process for such length blades approximately takes one third of the total production time, which comprises layup, infusion, and post-moulding assembly and other post-moulding operations. This allows for a continuous manufacturing process, where the mould system and post-moulding system is utilised at all times. However, for longer blades, such as blades having a length of 60 to 80 meters or even more, the layup time takes up a much larger part of the total production time, whereby the post-moulding system has a lot of idle time.
It is an object of the invention to obtain partly a new blade design and intermediary products of this design as well as a new method for manufacturing such wind turbine blades and intermediary products, and which overcomes or ameliorates at least one of the disadvantages of the prior art or which provides a useful alternative.